The present invention generally relates to a permanent magnet synchronous motor and, more particularly, to the synchronous motor generally used in a motor-driven compressor in a refrigerating system or an air conditioning system or any other industrially utilized electric appliance.
A self-starting permanent magnet synchronous motor operates as an inductor motor at the time of starting thereof owing to a starter squirrel cage conductor and as a synchronous motor as rotating magnetic poles created by the permanent magnets are entrained by a rotating magnetic field formed by a stator winding and moving angularly at a synchronous speed upon arrival of the rotor at a speed approaching the synchronous speed. This synchronous motor has an excellent constant speed operating performance and an excellent high efficiency. In particular, various improvement have hitherto been made to a rotor structure of the synchronous motor.
For example, the Japanese Patent Publications No. 59-23179 and No. 63-20105 discloses the prior art rotor structure for the self-starting permanent magnet synchronous motor.
FIG. 6 illustrates the prior art rotor disclosed in the Japanese Patent Publication No. 59-23179. Referring to FIG. 6, reference numeral 1 represents a rotor, and reference numeral 2 represents a rotor iron core having a plurality of slots 3 defined therein adjacent an outer periphery thereof. Conductor bars 4 are disposed within those slots 3 and have their opposite ends shortcircuited by respective shortcircuit rings to thereby form a starter squirrel cage conductor. The shortcircuit rings (not shown) are made of an annular electroconductive material disposed on axially opposite ends of the rotor iron core and are connected with the conductor bars 4. A plurality of magnet retaining holes 5 are provided on an inner side of the conductor bars 4, with corresponding permanent magnets 6 embedded therein. Reference numeral 7 represents magnetic flux shortcircuit preventive slits that are spaced such a small distance P from the magnet retaining holes 5 that magnetic saturation can take place between the magnet retaining holes 5 and the slits 7 to thereby prevent the magnetic fluxes emanating from the permanent magnets from being shortcircuited between the different magnetic poles.
FIG. 58 illustrates a longitudinal sectional view of the rotor used in the prior art self-starting synchronous motor disclosed in the Japanese Patent Publication No. 63-20105 and FIG. 59 illustrates a cross-sectional view taken along the line A-Axe2x80x2 in FIG. 58. Referring to FIGS. 58 and 59, reference numeral 11 represents a rotor, and reference numeral 12 represents a rotor iron core made up of a laminate of electromagnetic steel plates. Reference numeral 13 represents conductor bars having their opposite ends connected with respective shortcircuit rings 14 to thereby form a starter squirrel cage conductor. Reference numeral 15 represents permanent magnets embedded in the rotor iron core to form four rotor magnetic poles. Reference numeral 16 represents magnetic flux shortcircuit preventive slits each operable to prevent the magnetic fluxed between the neighboring permanent magnets of the different polarities from being shortcircuited. Reference numeral 17 represents an end plate disposed on each of axially opposite ends of the rotor iron core 2 by means of bolts to avoid any possible separation of the permanent magnets 5 from the rotor iron core 2.
When the prior art permanent magnet motor of the type provided with the cage conductor is to be used since the conductor bars and the permanent magnets are employed as rotatory drive elements, if the conductor bars and the permanent magnets are incorrectly positioned relative to each other, a force generated from the conductor bars and a force generated by the permanent magnets will be counteracted with each other and, therefore, no efficient rotatory drive will be achieved. Also, the permanent magnet motor provided with such a cage conductor requires a complicated and increased number of manufacturing steps since the permanent magnets and the conductor bars are provided in the rotor.
In view of the foregoing, the present invention is intended to solve those problems inherent in the prior art permanent magnet synchronous motor and is to increase the efficiency and simplify the manufacture of the synchronous motor of the type employing the permanent magnets.
To this end, the present invention according to a first aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars accommodated within corresponding slots defined in an outer peripheral portion of the rotor iron core, said conductor bars having their opposite ends shortcircuited by respective shortcircuit rings to form a starter squirrel cage conductor, said rotor having a plurality of magnet retaining slots defined therein at a location on an inner side of the conductor bars; and permanent magnets embedded within the magnet retaining holes in the rotor and defining rotor magnetic poles. In this synchronous motor, the neighboring members of the slots are spaced a distance which is referred to as a slot interval, the slot interval at a location adjacent one end of rotor magnetic poles being smaller than the slot interval at a location adjacent a center point of the rotor magnetic poles.
According to the first aspect of the present invention, the magnetic fluxes emanating from the permanent magnets will hardly leak to the outer peripheral surface of the rotor at a position adjacent opposite ends of the rotor magnetic poles and, instead leak to the outer peripheral surface of the rotor at a position adjacent a center point of the rotor magnetic poles. For this reason, the pattern of distribution of the magnetic fluxes in an air gap between the stator and the rotor represents a generally trapezoidal or sinusoidal waveform such that as compared with the rectangular waveform, the amount of change of the magnetic fluxes per unitary time increases and, therefore, the voltage induced across the winding of the stator can be increased to thereby intensify the rotor magnetic poles. Accordingly, in the practice of the present invention, to secure the required induced voltage, neither is the volume of the permanent magnets increased, nor the permanent magnets having a high residual magnetic flux density are required such as required in the prior art, thus making it possible to provide a high-performance and inexpensive self-starting synchronous motor having a required out-of-step torque and a high efficiency.
If the slot interval at a location spaced from the center point of the rotor magnetic poles in a direction conforming to a direction of rotation of the rotor is chosen to be greater than the slot interval at a location spaced from the center point of the rotor magnetic poles in a direction counter to the direction of rotation of the rotor, although during a loaded operation the maximum value of a distribution, on the rotor surface, of composite magnetic fluxes of the magnetic fluxes from the winding of the stator and the magnetic fluxes from the permanent magnets is positioned on one side conforming to the direction of rotation rather than the center point of the rotor magnetic poles, since the slot interval of the rotor through which the magnetic fluxes at that position pass is increased, the magnetic saturation at that portion can be prevented. Accordingly, the magnetic fluxes emanating from the magnets can be sufficiently taken from the rotor and, therefore, the current across the stator winding can be suppressed to thereby increase the efficiency of the motor.
The present invention according to a second aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars accommodated within corresponding slots defined in an outer peripheral portion of the rotor iron core, said conductor bars having their opposite ends shortcircuited by respective shortcircuit rings to form a starter squirrel cage conductor, said rotor having a plurality of magnet retaining slots defined therein at a location on an inner side of the conductor bars; and permanent magnets embedded within the magnet retaining holes in the rotor and defining rotor magnetic poles. In this synchronous motor, the slots have a radial length that is smaller at a center point of the rotor magnetic poles, and a distance between one of the slots positioned adjacent one end of the rotor magnetic poles and the magnet retaining holes is smaller than a distance between the slots positioned at other locations of the rotor and the magnet retaining holes.
According to the second aspect of the present invention, the magnetic fluxes emanating from the permanent magnets will hardly leak to the outer peripheral surface of the rotor at a position adjacent opposite ends of the rotor magnetic poles and, instead leak to the outer peripheral surface of the rotor at a position adjacent a center point of the rotor magnetic poles. For this reason, the pattern of distribution of the magnetic fluxes in an air gap between the stator and the rotor represents a generally trapezoidal or sinusoidal waveform such that as compared with the rectangular waveform, the amount of change of the magnetic fluxes per unitary time increases and, therefore, the voltage induced across the winding of the stator can be increased to thereby intensify the rotor magnetic poles. Accordingly, in the practice of the present invention, to secure the required induced voltage, neither is the volume of the permanent magnets increased, nor the permanent magnets having a high residual magnetic flux density are required such as required in the prior art, thus making it possible to provide a high-performance and inexpensive self-starting synchronous motor having a required out-of-step torque and a high efficiency.
Preferably, the distance between the slots in the rotor iron core and the magnet retaining holes progressively increases from a position adjacent one end of the rotor magnetic poles towards a position adjacent the center point of the rotor magnetic poles.
The present invention according to a third aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having two-pole windings wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars positioned adjacent an outer periphery of the rotor iron core, and shortcircuit rings positioned at axially opposite ends of the rotor iron core, said conductor bars and shortcircuit rings being integrally molded together by means of an aluminum die casting to form a starter squirrel cage conductor, said rotor having a plurality of magnet retaining slots defined therein at a location on the inner side of the conductor bars; and permanent magnets embedded within the magnet retaining holes in the rotor and defining two magnetic poles of different polarities. In this synchronous motor, the shortcircuit rings have an inner diameter positioned outside the associated magnet retaining holes, the inner diameter of the shortcircuit rings at a location adjacent one end of the magnetic poles being chosen to be greater than an inner diametric dimension at a location adjacent the center point of the magnetic poles.
According to this structure, the width of the permanent magnets can be increased and, therefore, with no need to increase the axial length of the permanent magnets, the requires area of surface of the magnetic poles of the permanent magnets can be secured. Accordingly, there is no need to laminate thickness of the rotor iron core, thereby decreasing the cost.
The inner diameter of the shortcircuit rings on one side where the permanent magnets are inserted may lie outside the magnet retaining holes in the rotor iron core, in which case the inner diametric dimension of one of the shortcircuit rings adjacent one end of the magnetic poles is chosen to be greater than the inner diametric dimension thereof adjacent the center point of the magnetic poles, and the inner diametric dimension of the other of the shortcircuit rings lies inwardly of the whole or a part of the magnet retaining holes. In this structure, an end plate made of a non-magnetizable plate is preferably positioned between such other shortcircuit ring and the rotor iron core so as to cover the magnet retaining holes.
This is particularly advantageous in that not only is there no need to increase the laminate thickness of the rotor iron, but also the cross-section of the other shortcircuit ring is increased to reduce the resistance, and therefore, the number of revolution of the motor at the time of a maximum torque can increase during a period the motor subsequent to the start thereof attains a synchronous speed, thereby increasing the starting performance of the motor.
Also preferably, the inner diameter of the shortcircuit rings on one side where the permanent magnets are inserted lies outside the magnet retaining holes in the rotor iron core, and the inner diametric dimension of one of the shortcircuit rings adjacent one end of the magnetic poles is chosen to be greater than the inner diametric dimension thereof adjacent the center point of the magnetic poles, whereas the inner diametric dimension of the other of the shortcircuit rings lies inwardly of the whole or a part of the magnet retaining holes. In such case, however, one or a plurality of electromagnetic steel plates of the rotor iron core adjacent the other shortcircuit ring is or are not formed with the magnet retaining holes.
The inner diameter of the shortcircuit rings on one side where the permanent magnets are inserted may be of a shape lying along the magnet retaining holes in the rotor iron core.
Where the stator iron core is made up of a stator laminate of electromagnetic steel plates and the rotor iron core is also made up of a rotor laminate of electromagnetic steel plates, the stator laminate has a thickness about equal to that of the rotor laminate.
The present invention in a fourth aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound and also having an inner cylindrical surface; a rotor including a rotor iron core in the form of a rotor laminate of a plurality of electromagnetic steel plates and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor iron core including a magnet retaining portion provided with magnet retaining slots, a magnetic flux shortcircuit preventive portion coupled with the magnet retaining portion and provided with magnetic flux shortcircuit preventive holes communicated with the magnet retaining holes, and a rotor outer end portion coupled with the magnetic flux shortcircuit preventive portion and provided with holes communicated with the magnetic flux shortcircuit preventive holes; and permanent magnets embedded within the magnet retaining holes in the rotor and defining rotor magnetic poles. In this structure, the magnetic flux shortcircuit preventive holes are smaller than the magnet retaining holes such that by allowing the permanent magnets to be held in engagement with outer edges of the magnetic flux shortcircuit preventive holes, the permanent magnets are axially positioned.
This structure is advantageous in that the axial position of the permanent magnets can be determined relying only on the rotor iron core and, accordingly, the cost required for assemblage and component parts can be reduced.
The present invention in a fifth aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core in the form of a rotor laminate of a plurality of iron plates and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor iron core including a magnet retaining portion provided with magnet retaining slots, and a permanent magnet support portion coupled with the magnet retaining portion and closing the magnet retaining holes; and permanent magnets embedded within the magnet retaining holes in the rotor and defining rotor magnetic poles. The permanent magnets being axially positioned by means of the permanent magnet support portion.
This structure is advantageous in that the axial position of the permanent magnets can be determined relying only on the rotor iron core and, since one ends of the magnet retaining holes can be closed by the rotor iron plate, closure of the magnet retaining hole by means of the end plate secured to the opposite ends of the magnet retaining holes is effective to permit the use of only one end plate to close the opposite ends of the magnet retaining holes.
An outer end of the rotor iron core may be coupled with the permanent magnet support portion and provided with hole positioned axially of the magnet retaining holes. In this case, the magnetic resistance of a magnetic circuit between the N and S poles at the axially opposite ends of the permanent magnets can be increased to reduce the leakage of the magnetic fluxes, resulting in increase of the motor characteristic.
Preferably, a starter squirrel cage conductor in the rotor iron core may be employed in the synchronous motor according to the fifth aspect of the present invention.
The present invention in a sixth aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars positioned adjacent an outer periphery of the rotor iron core and shortcircuit rings positioned at axially opposite ends of the rotor iron core, said conductor bars and said shortcircuit rings being integrally molded together by means of an aluminum die casting to form a starter squirrel cage conductor, said rotor iron core having a plurality of magnet retaining holes defined therein; and permanent magnets embedded within the magnet retaining holes at a location on the inner side of the conductor bars, said magnet retaining holes having a width in a radial direction of the rotor iron core being greater at a location inwardly of an axial direction of the rotor than at a location adjacent one end of the axial direction of the rotor.
According to this structure, even though shrinkage stresses generated as the shortcircuit rings after the aluminum die casting cools while undergoing shrinkage act on the ends of the rotor iron core, the gap between the permanent magnets and the magnet retaining holes can be maintained at a proper value and, therefore, the insertion of the permanent magnets into the magnet retaining holes can easily be attained, thereby securing a high-performance motor characteristics.
Where the width of the magnet retaining holes in the radial direction is smaller at opposite ends of the axial direction of the rotor than at a location inwardly of the axial direction of the rotor and further comprising an electromagnetic steel plate provided outside one of the opposite ends of the axial direction of the rotor for closing the magnet retaining holes, the use of only one end plate is sufficient and, therefore, the cost required for the end plate and the number of assembling steps can advantageously be reduced.
Also, where the width of the magnet retaining holes in the radial direction is greater at one of opposite ends of the axial direction of the rotor than at a location inwardly of the axial direction of the rotor and wherein the other of the opposite ends of the axial direction of the rotor is not provided with any magnet retaining holes for closing the magnet retaining holes at a location inwardly of the axial direction of the rotor, not only is the use of only one end plate sufficient, but also the number of combinations of the electromagnetic steel plates is minimized to form the rotor iron core, thereby facilitating manufacture of the motor having a high-performance.
The present invention in a seventh aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars positioned adjacent an outer periphery of the rotor iron core and shortcircuit rings positioned at axially opposite ends of the rotor iron core, said conductor bars and said shortcircuit rings being integrally molded together by means of an aluminum die casting to form a starter squirrel cage conductor, said rotor iron core having a plurality of magnet retaining holes defined therein; and permanent magnets embedded within the magnet retaining holes at a location on the inner side of the conductor bars. The rotor iron core employed is in the form of a laminate of electromagnetic steel plates and including an entwining portion provided adjacent the magnet retaining holes for lamination of the electromagnetic steel plates, and the magnet retaining holes adjacent the entwining portion has a width in a radial direction thereof which is partially enlarged in a direction towards the entwining portion.
According to this structure, even though when the entwining portion is formed by the use of any known press work, portions of the electromagnetic steel plates adjacent the entwining portion protrude under the influence of press stresses, the gap between the permanent magnets and the magnet retaining holes can be maintained at a proper value to thereby facilitate insertion of the permanent magnets and also to provide a high-performance motor characteristic.
The present invention in an eight aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars positioned adjacent an outer periphery of the rotor iron core and shortcircuit rings positioned at axially opposite ends of the rotor iron core, said conductor bars and said shortcircuit rings being integrally molded together by means of an aluminum die casting to form a starter squirrel cage conductor, said rotor iron core having a plurality of magnet retaining holes defined therein; and permanent magnets embedded within the magnet retaining holes at a location on the inner side of the conductor bars. The rotor iron core has conductor bar holes defined therein in an axial direction thereof and positioned inwardly of the magnet retaining holes, and the conductor bar holes are filled up by the aluminum die casting simultaneously with the starter squirrel cage conductor. The conductor bars so filled protrude a distance outwardly from an axial end of the rotor iron core to form respective projections for securement of an end plate. The end plate is made of a non-magnetizable material and secured fixedly to the end of the rotor iron core.
This structure is effective in that after the starter squirrel cage conductor and the projections for securement of the end plate have been formed simultaneously by the use of the aluminum die casting technique, engaging the projections into the engagement holes in the end plate and staking or crimping respective tips of the projections result in firm connection of the end plate to the end face of the rotor iron core and, therefore, with no need to employ any bolts, the end plate can easily be secured to the end of the rotor iron core. This permits reduction in cost for material and facilitates assemblage of the motor.
The end plate disposed at the axial end of the rotor iron core may be partly or wholly covered by the corresponding shortcircuit ring, in which case a job of connecting the end plate to the end face of the rotor iron core is sufficient at only one side of the rotor iron core.
The end plate covered by the shortcircuit ring may be provided with projections engageable in respective holes in the rotor iron core, so that positioning of the end plate can easily be performed and, also, the possibility can be eliminated which the end plate may displace from the right position under the influence of flow of a high-pressure aluminum melt during the aluminum die casting.
Also, one or a plurality of electromagnetic steel plates at one axial end of the rotor iron core may not be provided with any magnet retaining hole, in which case only one end plate is sufficient at the opposite axial end of the rotor iron core, thereby reducing the cost for material and the number of assembling steps.
In addition, projections may be provided at a location where the electromagnetic steel plates not provided with any magnet retaining holes contact the permanent magnets, so as to protrude towards the permanent magnets. In this case, the permanent magnets can be axially positioned upon engagement only with the projections and, therefore, the magnetic flux shortcircuit between the different poles of the permanent magnets through the electromagnetic steel plates can be reduced considerably, thereby increasing the performance of the motor.
The present invention in a ninth aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars positioned adjacent an outer periphery of the rotor iron core and shortcircuit rings positioned at axially opposite ends of the rotor iron core, said conductor bars and said shortcircuit rings being integrally molded together by means of an aluminum die casting to form a starter squirrel cage conductor, said rotor iron core having a plurality of magnet retaining holes defined therein, one of the shortcircuit rings having an inner periphery formed with recesses; permanent magnets embedded within the magnet retaining holes at a location on the inner side of the conductor bars; and an end plate made of a non-magnetizable material and having an outer periphery formed with projections complemental in shape to the recesses in the shortcircuit ring, a peripheral portion of each of the recesses in the shortcircuit ring being axially pressed to deform to thereby secure the end plate to an axial end of the rotor iron core with the projections in the end plate received in the corresponding recesses in the shortcircuit ring.
Thus, after the end plate can be mounted on the shortcircuit rings with the projections aligned with and received in the corresponding recess in the shortcircuit rings, pressing the respective peripheral portions of the recesses in the shortcircuit rings to deform results in fixing of the end plate to the end face of the rotor iron core, thereby facilitating the fitting of the end plate.
The present invention according to a tenth aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars positioned adjacent an outer periphery of the rotor iron core and shortcircuit rings positioned at axially opposite ends of the rotor iron core, said conductor bars and said shortcircuit rings being integrally molded together by means of an aluminum die casting to form a starter squirrel cage conductor, said rotor iron core having a plurality of magnet retaining holes defined therein, one of the shortcircuit rings having an inner periphery formed with recesses; permanent magnets embedded within the magnet retaining holes at a location on the inner side of the conductor bars; said magnet retaining holes being of a design allowing the permanent magnets, when embedded therein so as to be butted end-to-end in a generally V-shaped configuration to form a single magnetic pole, and having an air space defined between one end face of the permanent magnet and an inner face of one end of the magnet retaining hole for preventing shortcircuit of magnetic fluxes, a barrier slot for preventing shortcircuit of magnetic fluxes being defined between the magnet retaining holes for accommodating the neighboring permanent magnets of different polarities, a first bridge portion being provided between the magnet retaining hole and the barrier slot so as to sandwich the barrier slot, and a second bridge portion being provided between the neighboring permanent magnets of the same polarity and the corresponding magnet retaining holes, said second bridge portion being narrow at a location adjacent a center of the rotor and large at a location adjacent an outer periphery of the rotor.
This structure is effective not only to avoid shortcircuit of the magnetic fluxes between the different poles at the end faces of the permanent magnets to thereby increase the motor performance, but also to reduce the shrinkage strain of the rotor iron core outer diameter at the center of the rotor magnetic poles, that have resulted from shrinkage of the shortcircuit rings in a radial direction thereof after the aluminum die casting, to a very small value because of the strength of the bridge portion having been increased. Therefore, the gap size between the stator iron core inner diameter and the rotor iron core outer diameter can be accurately obtained merely by blanking the electromagnetic steel plates for the rotor iron core by the use of any known press work and the outer diameter of the rotor iron core need not be ground, thereby reducing the number of assembling steps.
The present invention according to an eleventh aspect thereof provides a synchronous motor which comprises a stator including a stator iron core having a winding wound therearound, said stator iron core having an inner cylindrical surface; a rotor including a rotor iron core and rotatably accommodated while facing the inner cylindrical surface of the stator iron core, said rotor including a plurality of conductor bars positioned adjacent an outer periphery of the rotor iron core and shortcircuit rings positioned at axially opposite ends of the rotor iron core, said conductor bars and said shortcircuit rings being integrally molded together by means of an aluminum die casting to form a starter squirrel cage conductor, said rotor iron core having a plurality of magnet retaining holes defined therein, one of the shortcircuit rings having an inner periphery formed with recesses; permanent magnets embedded within the magnet retaining holes at a location on the inner side of the conductor bars to provide two magnetic poles; said rotor iron core increasing from axially opposite ends thereof towards a center point of the length of the rotor to render it to represent a generally oval shape, the permanent magnets being mounted after formation of the starter squirrel cage conductor by means of the aluminum die casting.
According to this structure, even if the shrinkage strain of the rotor iron core outer diameter in a radial direction increases towards the center of the rotor magnetic poles after the aluminum die casting, the outer diameter of the rotor iron core after shrinkage can be kept to the right round shape and, therefore, the gap size between the stator iron core inner diameter and the rotor iron core outer diameter can be accurately obtained merely by blanking the electromagnetic steel plates for the rotor iron core by the use of any known press work and the outer diameter of the rotor iron core need not be ground, thereby reducing the number of assembling steps. Also, since the aluminum die casting is performed while the permanent magnets and the end plates have not yet been fitted, the job can easily be performed without incurring any defective component parts, thereby increasing the productivity.
Where the permanent magnets are employed in the form of a rare earth magnet, a strong magnetic force can be obtained and both the rotor and the motor itself can advantageously manufactured in a compact size and lightweight.